in ncdirect_render_image(), we want to emit newlines if and
only if we did not hit the right border. we want to emit
cuds (cursor downs) if and only if we've hit the bottom
border. this resolves all mysteries of blank lines, eliminating
a FIXME. it also fixes ncneofetch's image display on very
wide terminals. closes#756. also removes the display semaphore
in ncneofetch, using a more natural pthread_join().